This invention relates to an apparatus for detecting a relative displacement of a magnetic sensor comprising at least two magnetoresistive elements (hereinafter abbreviated as MR element) and a magnetic record medium having a magnetization pattern recorded thereon in a direction of displacement. Such an apparatus can be used as a rotary encoder, linear encoder, etc.
In the displacement detector comprising a plurality of MR elements, it has been known to effect a magnetic bias for the MR elements. For instance, in Japanese Patent Publication No. 37,204/78, it is proposed to arrange a MR element on respective sides of an insulating layer and to bias magnetically one of the MR elements by a magnetic field produced by a driving current passing through the other MR element and vice versa. Hereinafter this type of magnetic biasing is termed as a primary mutual bias system. In Japanese Patent Publication No. 37,205/78, there is disclosed another biasing method in which a magnetic field generated by a driving current passing through one of the MR elements is applied to the other MR element and one component of the magnetization in the other MR element produces a reverse magnetic field which is applied to the one MR element as a biasing magnetic field. This type of biasing is called a secondary mutual bias system. FIG. 1 shows a circuit diagram of a magnetic detector disclosed in the above mentioned Japanese Patent Publiation No. 37,204/78. First and second magnetoresistive elements MR.sub.1 and MR.sub. 2 arranged on respective sides of an insulating layer are connected in parallel with each other between a constant current source S and the earth potential and junction points between the first and second magnetoresistive elements MR.sub.1 and MR.sub.2 and the constant current source S are connected to inputs of a differential amplifier DA to derive a difference between voltages V.sub.1 and V.sub.2 at the junction points.
Such a magnetic detector of mutual bias type is formed by applying, on an insulating substrate, the first magnetoresistive element MR.sub.1, the insulating layer and the second magnetoresistive element MR.sub.2 successively. In order to produce a stable output signal, it is essential that the first and second magnetoresistive elements have the same magnetic characteristics. In a typical process for manufacturing such a magnetic detector, at first a first magnetoresistive film and a conductive film are applied on the substrate and then these films are shaped into a given pattern by means of a photoetching to form the first magnetoresistive element with a given conductor pattern. Next the insulating layer is applied and further a second magnetoresistive film and a second conductive film are successively applied on the insulating layer. Finally, the second magnetoresistive film and second conductive film are shaped into a given pattern by photoetching to form the second magnetoresistive element MR.sub.2 with a given conductor pattern. In such a manufacturing process, since the first and second magnetoresistive elements MR.sub.1 and MR.sub.2 are formed by the different magnetoresistive films, it is rather difficult to make various characteristics such as thickness, specific resistance, resistance-temperature coefficient and configuration coefficient of the first and second magnetoresistive elements equal to each other. Moreover, since the first and second magnetoresistive elements are formed by separate patterning steps, their dimensions are liable to be different from each other. In this manner, in the known magnetic detector, the first and second magnetoresistive elements have different magnetic characteristics and therefore, an unbalanced output voltage might be generated under a zero magnetic field and further the output voltage might drift due to temperature variation.
In case of detecting a rotation angle of a motor by means of the magnetic detector, the magnetic detector is not only exposed in a magnetic field caused by the magnetization pattern, but also exposed in a magnetic field caused by the motor. In this case, since it is difficult to select a signal magnetic field from a noise magnetic field, a signal to noise ratio of the output signal is decreased and detection accuracy becomes bad.